Add a acPreProcForExecCmd hook. 
It will be trigerred before executing a binary or script stored in server/bin/cmd through
iexecmd, msiExecCmd or the corresponding APIs.


git-svn-id: svn://irodssvn.ucsd.edu/trunk@5446 ea137bd6-aa86-47b6-b90d-b101d9f19a90